This invention relates generally to fuel vapor recovery systems and more particularly to fuel vapor recovery systems that capture fuel vapors during a refueling operation.
One such type of fuel vapor recovery system has a fuel vapor canister that captures fuel vapors and a refueling valve and one or more vent valves that are located at or near the top of the fuel tank for handling the fuel vapors in the fuel tank.
During the refueling process, a large volume of air and fuel vapors is displaced in the fuel tank by fuel. Thus when the refueling process is initiated, the refueling valve is open to provide large unrestricted passage for the air and fuel vapors to flow from the fuel tank to the vapor recovery system quickly during the refueling process. The refueling valve also automatically closes at least part way to restrict flow when the fuel in the fuel tank reaches a predetermined fill level to create a dome pressure above the fuel in the fuel tank which causes fuel to rise in the fill pipe and automatically shut off the fuel fill nozzle in a well known manner. After the refueling process is completed the dome pressure dissipates via the refueling valve until it is completely closed or a vent valve which allows the fuel level in the fuel fill pipe to recede.
The vent valve or valves on the other hand provide restricted passages from the fuel tank to the vapor recovery system which allow a low volume of fuel vapors to escape while guarding against liquid fuel contamination of the fuel vapor canister. The vent valves also close automatically when the fuel level in the fuel tank rises to a predetermined level.
A potential problem occurs in a fuel vapor recovery system of this type when a vent valve is located above the refueling valve so that the vent valve is open when fuel in the fuel reaches the fill level during the refueling process. Such an open vent valve can effect the dome pressure and delay the automatic shut off of the fuel filler nozzle so that the fuel tank may possibly be over filled.